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Toxicity to soil microorganisms

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Description of key information

The chemical safety assessment according to Annex I of Regulation (EC) No. 1907/2006 does not indicate the need to investigate further the toxicity to soil microorganisms.

Key value for chemical safety assessment

Additional information

No experimental data investigating the effects on soil microorganisms are available for the NPG esters. Therefore, all available related data is combined in a Weight of Evidence (WoE) approach, which is in accordance to the REACh Regulation (EC) No 1907/2006, Annex XI, 1.2, to adapt the data requirements of Regulation (EC) No 1907/2006 Annex VII - X (ECHA, 2012).

In the absence of a clear indication of selective toxicity, an invertebrate (earthworm or collembolan) test is preferred, as outlined in ECHA guidance section R.

All NPG esters are readily biodegradable and are thus expected to be removed from the terrestrial environment within a short period of time. A short-term study according to OECD 207 is therefore conducted with the smallest NPG ester Heptanoic acid, ester with 2,2-dimethyl-1,3-propanediol (CAS No. 68855-18-5) (log Koc < 5). Due to its smaller size and lower adsorption potential, this substance is assumed to represent the worst case for NPG esters in terms of highest bioavailability. The water solubility is negatively correlated with the C-chain of fatty acids (Lide, 2005). It was therefore used as read-across for all NPG esters. However, since the larger NPG esters have higher adsorption potential (log Koc > 5), a long-term study according to OECD 222 is conducted with one of the largest NPG esters 2,2-dimethyl-1,3-propanediyl dioleate (CAS No. 42222-50-4). This substance represents a worst case for the uptake via ingestion of soil particle bound substance and in terms of longest stability in soil. Summarising, NPG esters present in soil pore water will be readily degraded by microorganism, and long-term exposure is not to be expected. The short-term study is therefore considered sufficient in this case. For highly sorptive substances, which are not expected to be found in pore water (log Koc >5) a long-term study is available. However, these substances are readily biodegradable as well, so if they were to be found in pore water, removal via rapid biodegradation would take place. The two test substances thus represent both ends of the group, and the results will be used to cover all other NPG esters by interpolation.

Further justification is given within the category justification in IUCLID section 13.In addition, no effects were observed on activated sludge microorganisms with Heptanoic acid, ester with 2,2-dimethyl-1,3-propanediol (CAS No. 68855-18-5), which isthesmallest and thereby most bioavailable member of the NPG group.Moreover, in a chronic study on Daphnia magna, available for the NPG ester 2,2 -dimethyl-1,3 -propanediyl dioleate (CAS No. 42222 -50 -4), no long-term effects were observed at the test concentration of 1 mg/L, which is far above the water solubility of the NPG esters.

 The Guidance on information requirements and chemical safety assessment, Chapter R.7c: Endpoint specific guidance (ECHA, 2012) states that a test on soil microbial activity will only be additionally necessary for a valid PNEC derivation if inhibition of sewage sludge microbial activity has occurred, and this is clearly not the case. Since the NPG esters are readily biodegradable, they will be degraded quickly. Thus, acute tests with earthworm in combination with chronic aquatic data and toxicity data on microorganisms indicating no effects up to the limit of water solubility are sufficient to assess that the NPG esters have a very low toxicity to terrestrial organisms.

This is supported by further evidence from literature data. This data showed that soil microorganism communities are well capable of degrading fatty acid esters (Hita et al., 1996 and Cecutti et al., 2002) and use them as energy source (Banchio & Gramajo, 1997). Hita et al. investigated the degradation of the model molecule tristearin which is a triglyceride containing of glycerin tri-esterified with stearic acid in three different soils for 4 weeks. The amount of stearic acid increased in considerable amounts during the experiment showing the hydrolytic activity of lipases breaking the ester bonds. The investigation of ester fractions moreover showed the generation of new alkanoic acids (methyl stearate, ethyl stearate and propyl stearate) which were not determined in the controls. Nevertheless the amounts were no longer present after 4 weeks, which leads to the assumption that degradation by soil microorganisms had occurred. The same was shown by Cecutti et al. (2003). One soil sample was chosen and incubated with methyl oleate (plant oil) for 120 d. Methyl oleate and its metabolites were completely degraded after 60 d. Streptomyces coelicolor, a common gram-positive soil bacterium uses fatty acids (C4-C18) as sole carbon end energy source indicating that fatty acids are not-toxic and can be used for catabolism (Banchio and Gramajo, 1997). The available literature data shows that soil microorganisms are capable to break-up ester bonds and degrade fatty acids in significant amounts. Moreover, the data indicated the non-toxic properties of fatty acids since they can be used as energy source.

Based on the available data, terrestrial toxicity is not of concern for NPG esters.